Present invention relates generally to lamp controllers and lighting systems and more particularly to such controllers and lighting systems utilizing multiple intensities.
There are several common examples of multiple intensity lighting systems. Automobile headlamp systems have for many years operated on a dual intensity system. The headlamp system has a bright intensity for distant visibility and a dim intensity to protect oncoming vehicles from headlamp glare. Signalling systems which are observable in both daylight and nighttime have also used multiple intensities. It is desirable to have a brighter intensity in daylight for better visibility against a bright background. It is desirable to have a dimmer intensity in nighttime to protect from glare from the signal itself and to conserve energy since the lamp does not need to be as bright to be seen. A prime example where a multiple intensity signalling system is used is vehicular traffic intersection signals. Such traffic signals must be safely observable in bright light, yet not be obtrusive at night. Further, it is also desirable that the traffic signals not have an abrupt change in intensity. An abrupt change in intensity might be interpreted by drivers as a change in signal status.
Some multiple intensity lighting systems use multiple filament lamps in parallel. An example of this type of system is seen in U.S. Pat. No. 3,177,397, Keeran, Automatic Headlight Dimmer System, which utilizes one filament of a lamp for a high beam and a second filament of the lamp for a low beam. A switch selects between using either the high beam filament or the low beam filament. Similarly U.S. Pat. No. 3,244,934, Webb, Vehicle Signal System with Control of its Light Intensity, also uses dual filament lamps with the filaments connected in parallel. The system in Webb is designed for daylight and nighttime operation. The system utilizes filaments on multiple lamps in a series. However, to select multiple intensities, one of two parallel connected filaments is selected.
U.S. Pat. No. 4,095,100, Selick, Light Responsive Switch, discloses another means for utilizing multiple intensity lighting systems. Selick discloses a street light dimmer using an external resistance in series with a single filament lamp. The system effectively lowers the voltage across the filament. Note that in this system if the external dummy resistance fails, the lamp does not light. Further, the external dummy resistance dissipates a considerable amount of power.
Multiple intensity control systems may also vary the intensity of lamps by varying the power supplied to the lamp. This can be done by limiting the voltage applied to the filament, as with the external dummy resistance applied in Selick. U.S. Pat. No. 4,293,796, McMorrow, Traffic Light Dimming Technique and Circuitry, discloses a traffic light dimming control circuit which selects between either full-wave or half-wave power. Another means of varying the intensity of the power supplied to the lamp is the utilization of pulse width modulation of an alternating current supply to control the effective power supplied. U.S. Pat. No. 3,885,197, Moses, Light Dimmer, discloses a traffic light dimmer designed for daylight and nighttime operation with a single filament lamp. Moses uses pulse width modulation of the alternating current supply to control the dimming. The system tampers with the effective power supply delivered to the single filament lamp.
Other general light responsive traffic lamp dimming systems have been utilized to avoid the sudden shift between intensities. U.S. Pat. No. 3,500,455, Ross et al, Light Responsive Electrical Lamp Dimming Means, uses proportional dimming as a function of background illumination. The system involves dimming a single filament lamp. U.S. Pat. No. 3,500,456, Ross, Light Responsive Electrical Lamp Dimming Circuit, is similar, but also controls the intensity of a plurality of single filament lamps, also using a proportional dimming technique. U.S. Pat. No. 3,800,185, Anton et al, is similar to the Ross '455 patent, but uses a single gate control device to accomplish the dimming. U.S. Pat. No. 3,908,131, Anton et al, Control Circuit Having Load and Power Source Isolation, is similar to Anton '185, but is capable of controlling plural loads.